Hearing brains are ‘deaf’ to disappearance of sounds

28 September 2012

Our
brains are better at hearing new and approaching sounds than detecting
when a sound disappears, according to a study by researchers at the UCL Institute. The findings could explain why parents often
fail to notice the sudden quiet from the playroom that usually
accompanies the onset of mischief.

Hearing plays an important part as an early warning system to rapidly
direct our attention to new events. Indeed, we often rely on sounds to
alert us to things that are happening around us before we see them; for
example, somebody walking into the room while our back is turned to the
door. Yet little is known about how our brains make sense of the sounds
happening around us and what makes us hear certain events while
completely missing others.

Researchers at the UCL Ear Institute wanted to understand what makes
certain sounds easily detectable while others go unnoticed. They created
artificial 'soundscapes', comprising different ongoing sounds, and
asked listeners to detect the onset or disappearance of different
sound-objects within the melee.

Overall, the team found that listeners are remarkably tuned to
detecting new sounds around them but are much less able to detect when a
sound disappears. In busy sound environments, the participants missed
more than half of the changes occurring around them, and the changes
that were detected involved much longer reaction times. The effects were
observed even in relatively simple soundscapes and didn't seem to be
affected by volume.

Dr Maria Chait, who led the research, said:
"On the one hand, we might expect to be more sensitive to the appearance
of new events. In terms of survival, it is clearly much more important
to detect the arrival of a predator than one that has just disappeared.

Our results demonstrate that there are a large number of potentially urgent events to which we are fundamentally not sensitive. We refer to this phenomenon as 'disappearance blindness'.

Dr Maria Chait, UCL Ear Institute

"But this reasoning doesn't apply to other situations. Imagine
walking in a forest with your friend behind you and suddenly having the
sound of their footsteps disappear. Our results demonstrate that there
are a large number of potentially urgent events to which we are
fundamentally not sensitive. We refer to this phenomenon as
'disappearance blindness'."

The study also explored how resilient listeners are to scene
interruptions. In busy scenes, such as those we often face in the world
around us, important scene changes frequently coincide in time with
other events. The study showed that even brief interruptions, such as a
short 'beep' occurring at the same time as the change, are sufficient to
make listeners fail to notice larger scene changes. It is thought this
occurs because the interruption briefly captures our attention and
prevents the information about the change from reaching our
consciousness.

"Understanding what makes certain events pop out and grab attention
while others pass by unnoticed is not only important for understanding
how we perceive the world but also has important practical applications -
for example, to aid the design of devices intended to help
professionals such as air traffic controllers and pilots, who operate in
environments where the detection of change is critical," added Dr
Chait.

The study was funded by the Wellcome Trust and is published today in the journal PLOS ONE.